1. Field of the Invention
The present invention relates to a device for reading from a magnetic track. It more specifically aims at a magnetic read head of an electronic payment terminal or of a check reader.
2. Discussion of Prior Art
Some electric payment terminals are capable of reading cards provided with magnetic tracks, or magnetic cards. In this case, a magnetic card reader is provided, for example, in the form of a slot, open at its ends, where a user can slide a magnetic card. A magnetic read head, connected to a printed circuit of the terminal, is flush with an internal wall of the slot. The read head enables to read data stored on a magnetic track of the user's card on sliding of the card in the slot.
FIG. 1 is a front view schematically showing a magnetic read head 1 such as can be found in current electronic payment terminals.
FIG. 2 is a cross-section view of read head 1 along plane 2-2 of FIG. 1.
Read head 1 comprises an approximately parallelepipedal package 3, especially containing one or several electromagnetic read circuits. Package 3, generally made of metal, comprises a front surface 5 intended to be flush with an internal wall of the reader slot. Front surface 5 is slightly bulged, so that a magnetic card 7 (FIG. 2) can slide against this surface without risking to catch on package 3. Front surface 5 comprises one or several horizontal slots 8 (in the orientation of FIG. 1) defining reading paths, three paths 9a to 9c (FIG. 1) in this example, arranged at different heights to be able to read from magnetic tracks arranged at different heights on the card. This especially enables the reader to be compatible with different categories of magnetic cards. All reading paths are similar, for example, at the level of path 9b, package 3 contains a magnetic read circuit comprising a ring-shaped magnetic core 11b (FIG. 2), arranged along a plane parallel to the lower and upper surfaces of the package (in the orientation of FIG. 1). Ring 11b is not closed but is interrupted by a non-magnetic gap 13b, generally made of air or of a solid non-magnetic material such as brass or a resin. Gap 13b is in contact with or at very short distance from the portion of path 9b at the contact of which the magnetic card is intended to slide, and is generally covered with a protection varnish. A conductive winding 15b is formed around the region of the ring opposite to gap 13b. In practice, the height of gap 13b is slightly greater than the height of the magnetic tracks likely to be read. The gap width is linked to the data writing resolution on the magnetic track. In the field of magnetic payment cards, the gap width generally ranges between 5 and 50 μm.
Magnetic read circuits are also associated with reading paths 9a and 9c. FIG. 1 shows gaps 13a and 13c of the magnetic circuits associated with paths 9a and 9c. To improve the mechanical resistance of the head and/or for security reasons, the inside of package 3 is generally embedded in a resin (not shown) in a final manufacturing step.
The passing of a magnetic track of card 7 in front 13 of the concerned reading path 9 causes a variation of the magnetic flow in the corresponding core 11. This results in the occurrence of an induced current and thus in a variation of voltage VOUT across winding 15. Payment terminals generally comprise a processing unit, not shown, for extracting from signal VOUT the data stored on the magnetic track.
A disadvantage of this type of read heads is that they are very sensitive to external electromagnetic disturbances. Signal VOUT is thus tainted with noise. If the read head is used in a very noisy environment, it may become impossible to extract from signal VOUT the data stored on the magnetic card. This may especially occur in an electronic payment terminal located in a store, close to an anti-theft gate. In the case of a motor-driven magnetic check reader, the motors for driving the device also are a source of noise for the reader head. Further, the various wireless telecommunication devices are as many possible noise sources for read heads.
Some payment terminals comprise a software and/or hardware noise filtering unit. However, in addition to the extra cost resulting from the provision of such a unit, this solution does not enable to totally do away with external disturbances, especially when these disturbances are of high amplitude and/or generate parasitic signals in the same frequency band as the wanted signal.
Terminals comprising, in addition to the usual magnetic read head, an additional magnetic head for measuring the noise and a signal processing unit for restoring, from the signals provided by the read head and by the noise measurement head, an estimate of the non-noisy wanted signal, have also been provided. An embodiment of such a terminal, adapted to the magnetic reading of checks, is described in document FR-A-2898726. Such a solution has the disadvantage of resulting in a non-negligible extra cost and bulk, due to the presence of the additional measurement head and of the signal processing unit.